This application relates generally to fluid speed measurement and, more particularly, to a device for measuring the relative speed between a body and a fluid by measuring the shedding frequency from an obstacle placed in the flow. Particular applications of the device include the measurement of airspeed in flight vehicles and the measurement of the relative speed of hydrocraft through water.
Relative fluid speed measurements are required for aerodynamic and hydrodynamic control and, in the case of aircraft, for providing indicated airspeeds for stall speeds, landing speeds, never-exceed speeds, and true airspeeds for navigation, weapons release ballistics and a variety of other uses. Typically, aircraft speed is measured by pitot-static systems which provide indicated airspeed; that is, the ram or dynamic pressure compared to the static pressure for the particular altitude. (Similar measurements may be provided for watercraft using a pitometer.) The indicated airspeed provides information to the pilot for aerodynamic speed control to set such speeds as landing speed, best glide speed, maneuvering speeds and other aerodynamic speeds.
Traditional speed sensors such as pitot tubes, pitometers and impeller logs are highly subject to the effects of solid or liquid materials (e.g., dust, precipitation, insects or other organisms, ice particulates, and other materials (collectively referred to herein as “foreign material” or “foreign objects”). The effects of foreign objects on sensors and other equipment is typically referred to “foreign object damage,” or “FOD.” FOD can render such sensors inoperative or, potentially worse, inaccurate.
The susceptibility of prior art sensors to FOD is directly related to the fact that operation of these sensors requires that sensor elements being directly exposed to the impinging fluid. Such exposure, by necessity, results in exposing these elements to solid or liquid materials borne by the fluid. FOD can result due to impact, erosion, or material build-up, any of which can degrade or destroy the effectiveness or accuracy of the sensors.
Accordingly, it would be highly beneficial to provide a sensor that does not require direct exposure of the sensor element to the oncoming fluid flow and yet is capable of accurately measuring the speed of the fluid flow relative to the sensor and any object to which the sensor is attached.